Single Photon Emission Computed Tomography (SPECT) is a biomedical imaging technique that is often used in nuclear medicine to image functional processes. The gamma camera detects photons that are emitted due to decay of the tracer that was injected in the patient. The camera measures projections at different angles, which can then be used to reconstruct a 3D image of the distribution of the labeled molecules in the patient.
An important part of a SPECT scanner is the collimator. The collimator is used to only transmit gamma rays with certain directions. Behind this collimator is a detector that converts the gamma ray in a measurable signal. Two main types of collimators are known, being the parallel-hole collimator and the pinhole collimator. Pinhole collimators are used to select gamma rays from a cone. The projection of a pinhole collimator on a flat detector is a circular or elliptical region.
The efficiency of a pinhole collimator is typically very low because the aperture of a pinhole needs to be small in order to have a good resolution. To increase the efficiency of a pinhole collimator, multiple pinholes can be placed on the same collimator. To maximize the number of pinholes, an optimal usage of the detector area is necessary. Typically, a tradeoff has to be made when using pinholes on rectangular detectors. The projections of the different pinholes will either overlap, or some valuable detector area will not be used.
Overlap is often undesirable because the detections in the regions of overlap will be more ambiguous. However, a combination of overlapping and non-overlapping data can improve the reconstruction quality.
In the case of a full-ring detector, a cylindrical multi-pinhole collimator is typically used. The number of pinholes on the collimator is restricted by the size of the detector, the radius of the collimator, the field-of-view and the amount of overlap allowed on the detector. This restriction can be a problem for data-completeness, e.g. when the restricted amount of pinholes is less than 60. When the field-of-view is small, the detector is very large and/or the radius of the collimator is also large, data-completeness might not be a problem. In order to successfully reconstruct a 3D image, we need projections under different angles. 60 projections are typically used in clinical practice.